Insulin regulates the ability of the target cell to subsequently respond to insulin by modulating the number of insulin receptors as well as the activity of various post-receptor effector units. One of insulin's major biologic effects is to accelerate glucose transport, and this process involves rapid activation of already existing glucose transport carriers. On the other hand, it seems possible that insulin may also play a role in the longterm regulation, or maintenance, of the glucose transport system. Ample evidence drawn from in vivo studies exists to support this idea, and it is our hypothesis that the level of insulin to which cells are chronically exposed plays an important role in the long term regulation of the glucose transport system, distinct from insulin's effect to acutely activate the transport system. In this application we propose studies to characterize this process and to elucidate underlying mechanisms. In addition, it has been widely described that insulin can lead to a decrease in the concentration of its own cellular receptors. This phenomenon has been widely described both in vivo and in vitro. In the current application, we plan to define mechanisms whereby insulin leads to loss of cell surface insulin receptors following acute in vitro exposure to freshly isolated cells, and over the longer term in tissue culture, utilizing a variety of cell types (adipocytes, human fibroblasts, and IM-9 lymphocytes). We have also found that cells regenerate insulin receptors and that this is a process distinct from insulin induced receptor loss. Consequently, we will explore the mechanisms by which cells regenerate insulin receptors and attempt to understand the factors that regulate this process. Finally, we plan to correlate the relationship between changes in insulin receptors and glucose transport to the development of cellular insulin resistance or increased insulin sensitivity.